Micropattern-embossed oriented elastomer films

ABSTRACT

Elastomer film is extruded or coextruded against an engraved geometric micropattern roll which forms a geometric pattern of raised intersecting ridges or ribs on the film surface, and subsequently oriented in one or both directions to produce a heat-shrinkable, elastic film which retains its geometric micropattern embossment to an unexpectedly high degree, and which therefore has excellent non-blocking properties, reduced gloss and a satin appearance and feel.

This is a divisional of copending application(s) Ser. No. 07/637,359filed on Jan. 4, 1991, now U.S. Pat. No. 5,182,069.

OF THE INVENTION

1. Field of the Invention

The present invention relates to improved elastomer films, particularlyheat-shrinkable films, which are used for a variety purposes includingthe formation elastic waistband portions of disposable diapers.

2. Discussion of Prior Art

Diapers are produced from a variety of different plastic films which areused to form different portions of the diaper. The films used to formthe main portions, or backsheets of the diaper are inexpensivenon-elastomer thermoplastic films such as olefin homopolymers. Suchthermoplastic films conventionally are embossed during extrusion, butare not oriented, elastomeric or heat-shrinkable. Reference is made toU.S. Pat. Nos. 4,436,520 and 4,668,463 for their disclosure of embossingsuch films during extrusion.

Elastomer films are used to form small areas of disposable diapers,i.e., areas such as waistbands and legbands, where elastic propertiesare advantageous to improve the fit of the diaper. Such films generallyare oriented or stretched to impart the desired degree ofheat-shrinkability thereto so that shrinking occurs upon heatapplication.

Elastomer films generally present high blocking problems since theycontain adhesive polymers such as ethylene-vinyl acetate (EVA) and/orrubbers and oily plasticizers. Therefore it is known to extrude suchfilms against sandblasted embossing rolls to form a random micropatternembossment on a surface thereof to reduce the blocking tendencies of thefilm, whereby it can be rolled or fed more easily to the tenteringstation for orienting or stretching in the machine direction (MD) and/ortransverse direction (TD). However, the orientation draws down andsubstantially reduces the random surface embossment so that normal highblocking properties reappear. The high blocking properties make itdifficult to handle the film downstream of the tentering station, andmost difficult to unwind the elastomer film after it has been collectedon a roll.

It is known from U.S. Pat. No. 4,848,564 to extrusion-embossthermoplastic films such as polyethylene, and elastomeric films such ascopolymers of ethylene and vinyl acetate (EVA) or other co-monomers,using either a sandblasted or random micro-roughened embossing roll, oran embossing roll having its surface engraved with a geometric micropattern having an embossed depth from 0.0005" to 0 0025", suchgeometric-embossed rolls being described in U.S. Pat. No. 4,367,147.

The films of Pat. No. 4,848,564 are embossed in order to improve theirnon-blocking or release properties with respect to rubber bales on whichthey are used as wrappers. The embossed films of the patent are notoriented or stretched.

SUMMARY OF THE INVENTION

The present invention relates to the discovery of a novel process forproducing embossed and oriented, heat-shrinkable elastomer filmssuitable for use in diaper waistbands and other elastic structures inwhich the processing and handling of the films is simplified and thenecessity of feeding a normally-blocking or adhesive film through apost-embossing station is avoided.

The present invention relates to a process for orienting heat-shrinkableelastomer films which have been pre-embossed with a geometricmicropattern of particular dimensions which imparts properties ofreduced surface gloss, excellent feel and non-blocking to the film,which properties facilitate the handling of the film through anorientation station(s) and other downstream stations, and are retainedto an unexpectedly high degree after orientation of the film in one orboth directions.

The process of the present invention involves the step of producingembossed and oriented heat-shrinkable elastomer films, in which theelastomer film is extruded against or onto an engraved chill roll havinga fine geometric micropattern of predetermined dimensions, to produce anembossed elastomer film having low surface gloss, good feel andexcellent handling properties due to the resistance of the embossedsurface to block or adhere to other surfaces.

The latter property facilitates the subsequent steps of feeding of theembossed elastomeric film into the tentering or orienting station(s),and over bow rolls for flattening the web for collection on a roll, andsubsequent unwinding of the roll for final processing, which is the mostimportant advantage.

The most critical feature of the present invention resides in the use ofan extrusion-embossment means comprising a geometrically-engraved maleor female micropattern surface having from about 50 to 350, preferably amale micropattern having about 100 to 250, most preferably about 165,repeating geometric units or lines per inch, measured in the transversedirection (TD) and/or machine direction (MD). The male micropattern isformed by intersecting embossed or engraved lines, in the number statedabove, each line having a depth within the range of 0.001" to 0.0025",most preferably about 0.0018" or 18 ten thousandths of an inch.

DETAILED DESCRIPTION OF THE INVENTION

The elastomeric films suitable for processing according to the presentinvention include all thermoplastic elastomers or rubberythermoplastics, including blends of a thermoplastic material and anelastomer which are processable as a melt, at elevated temperatures, andexhibit properties similar to vulcanized elastomers at room temperature.

The preferred thermoplastic elastomer film compositions for useaccording to the present invention comprise three main components, (1)olefinic elastomer, (2) ethylene copolymer and (3) a hydrocarbon processoil.

The concentrations of the three main components of the blend are asfollows:

    ______________________________________                                                      Concen-  Preferred   Most                                       Component     tration  Concentration                                                                             Preferred                                  ______________________________________                                        Olefinic Elastomer                                                                         10-40 wt %                                                                              15-30 wt %  20-30 wt %                                 Ethylene Copolymer                                                                         40-90 wt %                                                                              50-80 wt %  60-75 wt %                                 Process Oil   0-15 wt %                                                                               2-12 wt %   4-10 wt %                                 ______________________________________                                    

The above concentration ranges may be combined in any permissiblecombination, although particular combinations are shown as preferred andmost preferred.

Elastomer Component: The olefinic elastomer component of the compositionpreferably comprises an ethylene copolymer elastomer, such as acopolymer of ethylene with higher alpha-olefin. Preferred ethyleneelastomer copolymers include EPM (ASTM D-1418-72a designation for anethylene-propylene elastomer copolymer) or EPDM (ASTM D-1418-72adesignation for an ethylene-propylene diene elastomer terpolymer). Alsousable are high molecular weight polyisobutylene, butyl rubbers andhalogenated butyl rubbers.

Preferred ethylene elastomer copolymers for use herein comprise from 30to 90 percent ethylene, more preferably from 35 to 80 weight percentethylene, and most preferably from 50 to 80 weight percent ethylene andhave a Mooney viscosity 40-60 (ML 1+4 at 257° F.).

EPDM is a terpolymer of ethylene, a higher alpha-olefin such aspropylene, and a non conjugated diene. In such elastomers, thenonconjugated diolefin may be straight chain, branched chain or cyclichydrocarbon diolefins having from 6 to 15 carbon atoms.

Of the nonconjugated dienes typically used to prepare these copolymers,preferred are dicyclopentadiene, 1,4-hexadiene, 5-methylene-2-norborneneand 5-ethylidene-2-norbornene; 5-ethylidene-2-norbornene (ENB) and 1,4-hexadiene are particularly preferred diolefins. EPDM elastomers andtheir method of manufacture are well known to those skilled in the art.Oil extended EPDM elastomers may also be used. Preferred EPDM elastomerscontain from 30 to 90 weight percent ethylene, and from 0.5 to 15 weightpercent of the nonconjugated diolefin.

As mentioned above, the olefinic elastomer useful in this invention mayalso be polyisobutylene, a copolymer of isobutylene and isoprene(generally known as butyl rubber) or a halogenated copolymer ofisobutylene and isoprene (generally known as halogenated butyl rubber,such as chlorinated, brominated and chlorobrominated butyl rubber).Butyl rubber is vulcanizable rubber copolymer containing from 85 to 99.5percent combined isolefin having from 4 to 8 carbon atoms and from 0.5to 15 percent combined conjugated diolefin having from 4 to 8 carbonatoms. Such copolymers and their preparation are well known, andgenerally the isoolefin is a compound such as isobutylene and thediolefin is compound such as butadiene or isoprene. Halogenated butylrubbers are also well known: chlorinated and brominated butyl rubbergenerally contains from 1.0 to 3.0 weight percent bromine and from 0.05to 0.5 weight percent chlorine.

Ethylene Copolymer Component: The ethylene copolymers include those ofethylene and alpha-olefins having 3 to 16 carbon atoms such as propyleneor 1-butene. Also included are copolymers of ethylene with anunsaturated carboxylic acid. In particular, copolymers of ethylene withvinyl acetate (EVA), or with acrylic acid (EAA), or methacrylic acid(EMA), are preferred. The ethylene copolymers to be employed generallycontain from 50 to 99 weight percent ethylene, most preferrably from 60to 95 percent ethylene.

The most preferred ethylene copolymer useful in the present invention isEVA. The EVA may have a vinyl acetate (VA) content between about 9% and40% by weight, with about 15 to 35 weight percent VA being preferred.

VA contents below about 9 wt % do not possess sufficient flexibility andorientability for purposes of the present invention and VA contentsabove 40 wt% exhibit excessive tackiness. The best balance oforientability and non-tackiness occurs at the VA contents between 15 and35 wt%.

Preferred Melt Index (ASTM-D-1238, Condition E) for EVA is from 1 to 20,with 2 to 10 being most preferred.

The ethylene copolymer component normally will determine the operatingtemperatures of the tentering and annealing operations. These operationsmay be carried out at temperatures not less than 100° F. and below thecrystalline melting point of the ethylene copolymer component. Thecrystalline melting point of EVA ranges from approximately 105° F. and200° F., depending on the VA content and MI, with the preferred EVA'shaving crystalline melting points between about 130° F. and 160° F. Foreconomic reasons, orienting temperatures of 160° F. and below arepreferred.

Process Oil Component: Hydrocarbon oils useful in the present inventionfunction as process aids whose activity is enhanced in the presence ofvinyl acetate copolymers, as plasticizers producing low modulus andenhanced elasticity in the solid state. Useful oils are the normallyliquid hydrocarbon processing and extender oils (ASTM D 2226)categorized as aromatic, highly aromatic, naphthenic and paraffinicprocess oils of a medium viscosity range. Oils sold under the trademarks"Flexon" and "Arco Prime" have been found especially useful.

Other Additives: The composition may also include pigments, a fillermaterial, an antiblock agent, processing aids, stabilizers and otherconventional additives.

FILM PREPARATION

Resin/Blend Preparation: Preparation of compositions usable in thisinvention can be achieved in several different ways. The variouscomponents may be brought into intimate contact by, for example, dryblending the components and additives and then passing the overallcomposition through a compounding extruder. Alternatively, thecomponents may be fed directly to a mixing device such as a compoundingextruder, high shear continuous mixer, two roll mill or an internalmixer such as a Banbury mixer. The optional ingredients previouslydescribed can be added to the composition during this mixing operation.Overall, the objective is to obtain a uniform dispersion of allingredients and this is readily achieved by inducing sufficient shearand heat to cause the plastics component(s) to melt. However, time andtemperature of mixing should be controlled as is normally done by oneskilled in the art so as to avoid molecular weight degradation.

Film Extrusion: Film from the resin compound is manufactured byconventional cast extrusion. The molten resin is extruded from acoathanger die in the form of a web which is then cast onto ageometrically-etched, micropatterned, embossing chill roller, whichsolidifies the polymer. The embossed web is moved directly into thetentering station for orientation.

The extrusion temperatures, die temperatures, and chill rolltemperatures will depend on the composition employed, but generally willbe in the following ranges for the compositions of the present inventionprepared by cast extrusion:

    ______________________________________                                        Melt Temperature (°F.)                                                                     350-450                                                   Die Temperature (°F.)                                                                      350-450                                                   Chill Roll Temperature (°F.)                                                                70-130                                                   ______________________________________                                    

The process described above may include one or two embossing rolls tochill and emboss the film on one or both surfaces.

In practice, the process of the present invention is carried out usingan in-line operation wherein the extruder and orientation system (e.g.,tenter) are arranged in tandem to form the film by casting or meltembossing followed by film orientation.

In a preferred embodiment, the compounded extrudable compositioncontaining the elastomer ingredients along with the other additives isintroduced into an extruder and extruded into a web from a flat die andmelt-embossed through counter rotating chill and embossing rolls. Thefilm thickness may vary from 50 to 400 microns before orientation.Preferably the film will have a final stabilized thickness of between 10and 100 microns after orientation and annealing.

Since some thermoplastic elastomeric films are quite tacky and presentblocking problems which make them difficult to handle and feed, such asthe preferred elastomers discussed above, it is preferable to co-extrudesuch elastomers as a core layer between surface or cap layers ofthermoplastic elastomers having reduced blocking properties, generallydue to a lower content of adhesive vinyl acetate in the ethylene-vinylacetate (EVA) component and the absence of the process oil component.

It is important that the surface layer(s) be sufficiently elastic andthin to avoid interference with the higher shrink and elastic propertiesof the thermoplastic elastomer.

As formed on the core or base layer, each of the surface layer(s) willcomprise between 2 to 15, preferably 2 to 10 weight percent of thecomposite and not more than 25 microns thick. Following the orientation,the surface layer(s) will be reduced to a thickness of not more than 7microns and as thin as possible. Preferred thickness after orientationis between 1 and 5 microns.

The surface layer may be coextruded onto only one side of the core layerbut preferably is extruded onto both sides.

The critical extrusion-embossment step is accomplished by extruding thethermoplastic elasomer layer, or co-extruding an AB or ABA structure ofa core layer B having exceptional elasticity and heat-shrink properties,and one or more surface or cap layers A of thermoplastic elastomericcomposition having lower tack and lower blocking properties than thecore layer. The extruded layer or coextrudate is passed in the form of acontinuous molten web in the nip between the geometrically-engravedmicropattern surface of a chilled embossing roll and a support rollwhich is coated with rubber, silicone or other release surface, orbetween two similar chilled embossing rolls.

After embossment, the film is moved to an in-line tenter or orientingdevice. Orientation of the film may be carried out in the machinedirection (MD) or the transverse direction (TD) or both directions(biaxially) using conventional equipment and processes.

For orientation in the MD, the embossed film at an elevated temperature(but below the crystalline melting point of the polymer) is passedaround two rollers driven at different surface speeds, over a pair ofbow rolls to flatten the film, and finally to a takeup roller. Thedriven roller closest to the takeup roll is driven faster than thedriven roller closest to the feed roll, such that the film is stretchedbetween the driven rollers. The assembly may include a rollerintermediate the second roller and takeup roller to further cool thefilm. If supplementary cooling is not used, the film will cool toambient temperature on the take up roll.

The degree of stretch, generally between 50% and 500%, will depend uponthe relative peripheral speeds of the driven rollers and the distancebetween the rollers. Stretch rates of 50 to 500 percent/minute will besatisfactory for most MD orientation applications.

Preferably, however, film orientation is carried out in a tenteringdevice to impart TD orientation to the film. The film is extrusionembossed as described above and then gripped by the edges for processingthrough the orientation steps. The film is passed successively through apreheat step, a stretching step at elevated temperatures (e.g. from 100°F. to a temperature slightly below the crystalline melting point of theethylene copolymer), an annealing step, and finally a cooling step.(Although cooling may be considered part of the annealing step, forconvenience it is described as a separate step herein.) The preheat,orientation, and a portion of the annealing temperature is controlled atan elevated temperature but below the crystalline melting point of thepolymer. Although not essential, it is preferred that tension bemaintained on the film during the annealing and cooling steps to preventsag and to minimize shrinkback. Upon cooling to ambient temperature(i.e., room temperature) or near ambient, the holding force may bereleased. The film may contract somewhat (snapback) in the TD but willretain a substantial portion of its stretched width.

The tenter operating conditions can vary within relatively wide rangesand will depend on the several variables including film composition,film thickness, degree of orientation desired, annealing conditions,etc. The following is exemplary of a process for stretching 100 micronthick ABA coextruded film (containing EVA) from 43 inches wide to afinal width of about 88 inches, using a tenter manufactured by Marshalland Williams Company of Providence, R.I.

    ______________________________________                                        ESTIMATED FILM RANGE                                                                                              Approximate                               Step    Broad     Preferred Typical Time (Sec.)                               ______________________________________                                        Preheat 100-160 F 130-160 F 155 F   3.0                                       Stretching                                                                            100-160 F 115-145 F 135 F   9.0                                       Annealing                                                                             100-160 F 110-140 F 120-130 F                                                                             3.0                                       Cooling Ambient   Ambient   Ambient 6.0                                       ______________________________________                                    

As indicated earlier, it is highly desirable to employ an annealing stepin the process. Annealing partially relieves the internal stress in thestretched film and dimensionally stabilizes the film for storage. It hasbeen found that by annealing the film at a temperature of +40° F.,preferably +20° F. of the orientation temperature (but slightly belowthe crystalline melting point of the ethylene copolymer), eliminatesundesirable shrinkage during storage. The preferred annealingtemperature is between 110° F. and 140° F. Temperatures which result inexcessive stress relieving should be avoided, since substantialfrozen-in stresses and strains should remain after the process iscompleted.

Annealing can be accomplished by maintaining the film in thenearly-completely stretched condition at the annealing temperature.Preferably, the annealing and cooling is carried out while permittingthe film to contract slightly, but still under stress. The guide railsof the tenter can be arranged in a converging manner to provide theannealing and cooling while the film contracts. The controlledshrinkback of from 5 to 30%, preferably between 10 and 25%, of themaximum stretched width has given particularly good results ineliminating storage shrinkage. This annealing and preshrinking removessome of the film stresses and strains so that shrinkage will not occurat storage temperature. However, the annealing and cooling does notremove all the frozen-in stress and strain, since upon heating toelevated temperatures above storage temperature the film will shrink.

The degree of stretching may vary within wide ranges. Draw ratios of1.3:1 to 6:1 are possible with 2:1 to 4:1 being preferred for TDtentering. Orientation in the transverse direction only, and to anextent of between 200% and 400%, is preferred.

Films produced according to the simplified in-line extrusion-embossmentand orientation process of the present invention retain to anunexpectedly high degree the aesthetic and functional propertiesimparted during the geometric embossing step, i.e., a micropattern mattesurface which has the appearance and feel of satin and which hasexcellent non-blocking properties which facilitate the unwinding of thefilm from a roll and substantially improve the handling properties ofthe film for final processing into diaper waistbands and/or otherstretchable components or products.

These results are obtained through the use of the geometricmicropattern-engraved embossing rolls identified herein, having thestated number of intersecting lines or cut valleys per inch, each havinga depth between about 0.001 and 0.0025 inch, forming between said linesmale pins or uncut peak areas having heights between about 0.001 and0.0025 inch. Random-engraved or sandblasted embossing rolls produce amatte surface on the film but the appearance and functional propertiesthereof are minimized or degraded during the orientation or tenteringstep, so that the final film has a high-gloss surface which does nothave the appearance or feel of cloth and has high blocking or adhesionproperties.

An apparent explanation for the unexpected advantages obtained from theuse of the present geometric embossing rolls in the present process isthat a geometric embossing surface comprising geometrically-aligned pinsor peak areas forms a geometric repetitive pattern of depressions,valleys or recesses in the elastomer surface surrounded by intersectingand connected peak lines, ridges or ribs which provide a skeletalsupport network during orientation. Such structure uniformly relieves orabsorbs the orientation stresses, and the raised portions, peaks, ridgesor ribs of the film surface are retained. In other words, the geometricvalleys or depressions in the film surface are widened somewhat duringorientation while the surrounding raised peak areas, forming theuppermost surface of the film, are stretched and drawn down somewhat sothat the film surface retains its embossed appearance and feel and itsnon-blocking properties.

It is to be understood that the above described embodiments of theinvention are illustrative only and that modifications throughout mayoccur to those skilled in the art. Accordingly, this invention is not tobe regarded as limited to the embodiments disclosed herein but is to belimited as defined by the appended claims.

I claim:
 1. A geometric-pattern-embossed and oriented, heat-shrinkable,elastic, thermoplastic elastomer film produced by an in-line process,said film comprising at least one thermoplastic elastomer, saidthermoplastic elastomer being a blend of an olefinic elastomer, anethylene copolymer, and a process oil, said thermoplastic elastomerbeing extrusion-embossed with a geometric micropattern surfacecomprising between about 50 and 350 lines per inch, each having a depthbetween about 0.001 and 0.0025 inch and forming a geometric pattern ofrecessed surface areas and surrounding, intersecting raised surfaceareas on said film, and said film is oriented in-line in the transversedirection and/or the machine direction to an extent between about 50%and 500%, whereby the film is uniformly stretched in the geometricrecessed areas and in the geometric intersecting raised areas of thefilm, surrounding said recessed areas, whereby said film substantiallyretains the effects of the embossment, after orientation.
 2. A Filmaccording to claim 1 in which said ethylene copolymer comprises anethylene-vinyl acetate copolymer having a vinyl acetate content fromabout 9 to about 40 weight percent.
 3. A film according to claim 1 inwhich said thermoplastic elastomer comprises an ethylene-propylenepolymer, an ethylene-vinyl acetate copolymer, and hydrocarbon processoil.
 4. A film according to claim 1 which comprisesa co-extrudate ofleast two different extrudable compositions, each comprising at leastone thermoplastic elastomer, comprising a core layer of one elastomercomposition having high blocking properties and at least one surfacelayer of a different elastomer composition having lower blockingproperties.
 5. A film according to claim 4 which comprises said corelayer of one elastomer composition, and inner and outer surface layersof said different elastomer composition.
 6. A film according to claim 5which is embossed only on one of said surface layers.
 7. A filmaccording to claim 1 in which said embossed geometric micropattern onsaid film surface comprises between about 100 and 250 lines per inch,each having a depth between about 0.001 and 0.0025 inch.
 8. A filmaccording to claim 1 which is stretched only in the transversedirection, and only to an extent between about 200% and 400%.